The physics core is responsible for the maintenance of the positron emission tomograph and its associated computer systems, the development of image reconstruction, display and analysis software, and the development of methods to improve the quality and quantitative accuracy of the image data. The current tomograph is a University of Washington modified PETT Electronics SP-3000 time-of-flight system. It includes 4 rings of detectors to produce 7 image planes (14 with detector array translation). The tomograph data processing system utilizes a Data General MV 18000SX for data acquisition, a Data General A ViiON 6240 for image reconstruction, and Data General A ViiON 6220 and A V300 systems for data analysis and software development. The tomograph provides a limiting resolution of 5 mm, a slice thickness of 11.5 or 7.5 mm (operator selected), and a sensitivity of 7,500 cps/mu-Ci/cc for direct image planes and 8,900 cps/mu-Ci/cc for cross planes in the preprocessed mode (4.8 ns timing window). The machine can produce coincidence count rates of greater than 100,000 events/second/image plane. The field of view is 45 cm with a patient tunnel diameter of 50 cm. The physics group is also in the process of selecting a second PET system to be installed in 1993. This system will be chosen to complement the existing tomograph by providing a larger axial field of view with much finer axial sampling. The system will be optimized for high resolution, moderate count rate studies. The facilities provided by the core include more than the basic tomograph. The host computer system used for data processing and software development is used by all of the projects and cores. The software developed by the core is used on the primary computer systems and on the Macintosh II workstations being utilized by the project investigators. Thus, the physics core affects all of the projects and the imaging core (core C). A major effort will be made to further reduce the image reconstruction and analysis time. Our current software will be modified to take full advantage of the four processor environment of the A ViiON 6240 system. Our image analysis software will be modified to allow close integration between the workstations and the central computing resources. Software for kinetic modeling and generation of parametric images will be fully integrated into our workstations and central computing resources (in collaboration with Core C). Image accuracy and quality will be improved with: new methods for attenuation scans (implementing accidental event and scatter rejection); optimization of energy discriminators on the tomograph; changes in the basic image reconstruction algorithms (new methods for accidental event and spatially variant scatter corrections, revisions in the confidence-weighted backprojection reconstruction algorithms, etc.); implementation of image registration techniques; and investigation and implementation of partial volume corrections. Finally, the second PET system will be fully integrated into our existing data analysis systems.